A low dispersion glass is a glass useful for aberration correction of an optical element. In an imaging system lens typically used in a digital camera, there is a growing demand for decreasing the number of lenses to miniaturize an optical system, and a glass molded lens which has been precision-pressed into an aspherical shape is in heavy usage. For aberration correction, lower dispersion is preferred.
In order to achieve low dispersion, there is a technique of introducing fluorine into a glass. However, in such technique, there have been problems that since fluorine providing an environmental load vaporizes at the time of glass melting, an apparatus for removing fluorine is required, striae occurs by deviation in composition caused by the vaporization of fluorine, or it becomes difficult to mold a glass because of its extreme low-softening due to the introduction of fluorine.
In view of the above and production cost, an oxide glass is preferred. Consequently, studies of phosphate-barium glass which is expected to achieve low dispersion among the oxide glass have been made (Patent Documents 1 to 4). From the viewpoint of compactification of the lens, it is desirable that a glass has a high refractive index together with low dispersion. However, there has been no glass having sufficient low dispersibility (νd≧63) in a specified high refractive index region (nd≧1.59), and being excellent in melt stability, press moldability and chemical durability.    Patent Document 1: JP-B-63-20775    Patent Document 2: JP-A-2005-8518    Patent Document 3: JP-A-2007-15904    Patent Document 4: JP-A-2006-111499